Magnetic resonance imaging (MRI) systems have become ubiquitous in the field of medical diagnostics. Over the past decades, improved techniques for MRI examinations have been developed that now permit very high quality images to be produced in a relatively short time. As a result, diagnostic images with varying degrees of resolution are available to the radiologist that can be adapted to particular diagnostic applications.
In general, MRI examinations are based on the interactions among a static magnetic field, a radio frequency (RF) magnetic field and time varying magnetic field gradients with nuclear spins within the subject of interest. The nuclear spins, such as hydrogen nuclei in water molecules, have characteristic behaviors in response to external magnetic fields. The precession of such nuclear spins can be influenced by manipulation of the fields to obtain RF signals that can be detected, processed, and used to reconstruct a useful image.
The magnetic fields used to produce images in MRI systems include a highly uniform, static main magnetic field that is produced by a magnet. A series of gradient fields are produced by a set of three coils disposed around the subject. The gradient fields encode positions of individual volume elements or voxels in three dimensions. A radio frequency coil is employed to produce an RF magnetic field. This RF magnetic field perturbs the spin system from its equilibrium. Upon returning to the equilibrium after the termination of the RF field, an RF signal can be emitted. Such emissions are detected by either the same transmitting RF coil, or by a separate receive-only coil. These signals are amplified, filtered, and digitized. The digitized signals are then processed using one of several possible reconstruction algorithms to form a final image.
Many specific techniques have been developed to acquire MR images for a variety of applications. One major difference among these techniques is in the way gradient pulses and RF pulses are used to manipulate the spin systems to yield different image contrasts, signal-to-noise ratios, and resolutions. Graphically, such techniques are illustrated as "pulse sequences" in which the pulses are represented along with temporal relationships among them. In recent years, pulse sequences have been developed which permit extremely rapid acquisition of a large amount of raw data. Such pulse sequences permit significant reduction in the time required to perform the examinations. Time reductions are particularly important for acquiring high resolution images, as well as for suppressing motion effects and reducing the discomfort of patients in the examination process.
Particularly dramatic reductions in acquisition time have been obtained through a technique referred to as echo planar imaging (EPI). In EPI, a bi-polar gradient waveform is used concurrently with the echo-train acquisitions. Each echo in the echo train is individually phase-encoded, usually by a small blip gradient pulse, to produce a line of k-space data. In doing so, multiple k-space lines can be acquired in a single excitation. The k-space lines acquired in a single excitation can be used to reconstruct an image, a technique known as "single-shot EPI." Alternatively, the k-space lines from multiple excitations can be combined for image reconstruction, a technique known as "multi-shot EPI." The single-shot technique can provide excellent temporal resolution (less than 100 ms), whereas multi-shot EPI can be used to improve spatial resolution.
In another examination type, known as diffusion-weighted (DW) imaging, pulse sequences are specifically adapted to provide contrast between molecules having different degrees of freedom of movement. In this type of imaging, diffusion-weighting gradients are produced which sensitize resulting signals to the small random movements of molecules. Contrast can be provided in this manner between molecules which are relatively free to diffuse, such as cerebrospinal fluid, and those which are relatively more constrained, such as tissues with densely packed cells.
Diffusion-weighted imaging has been combined with EPI techniques to provide useful tools for specific clinical applications. In particular, DW-EPI techniques have been successfully used for early detection of cerebral ischemia. The DW-EPI technique is often preferred due to its effectiveness in suppressing motion artifacts as may result from bulk patient motion. In DW-EPI sequences, a pair of diffusion-weighting gradient lobes with high amplitude straddle a 180.degree. refocusing RF pulse. The diffusion-weighting gradient sensitizes the MR signals to the diffusion of water molecules which can be subsequently used as a contrast mechanism to distinguish different tissues.
Due to the high amplitudes of the gradient lobes employed in diffusion-weighted imaging, disturbances resulting from eddy currents can be produced. Such eddy currents generally arise from the changes in the magnetic field associated with the diffusion-weighting gradient pulses. These magnetic fields can induce currents in certain conductive materials in the vicinity of the gradient coils, such as thermal shields in the magnet bore. Although the behavior of such eddy currents can be modeled or analyzed when eddy current exceed certain threshold, minute residual eddy currents are difficult to measure directly, making modeling and analysis impractical. Such minute residual eddy currents produce perturbations in the magnetic field system that can result in image artifacts such as shift, distortion, and intensity reduction in DW-EPI images.
Techniques have been developed for compensating for eddy current effects in DW-EPI. One such technique is described in U.S. Pat. No. 5,864,233, assigned to the assignee of the present invention.
While the compensation technique produces good results, it requires a priori knowledge of the time constants and amplitudes of the residual eddy currents, which are difficult to obtain in the presence of noise, as stated above. Thus, there remains a need for a compensation technique that does not rely on the knowledge of the time constants and amplitudes. In particular, there is a need to obtain the eddy current compensation parameters from an empirical measurement that faithfully characterize the residual eddy current status of the MR system. By applying the compensation parameters thus derived to the DW-EPI pulse sequences, the new compensation technique should considerably reduce, or even eliminate, the image quality problems in DW-EPI, including image shift, distortion, and intensity reduction.